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Hi, my investigation is on the effect of vitamin C concentration over a range of temperatures in orange juice. First of all how is vitamin C destroyed in the heating process and what does it become? Also the technician in my school told me to put the orange juice in the burette because if you put it into the beaker it will oxidise because it is exposed to air. But I can't see any anywhere that tells you to do this? Why do you put the DCPIP in the burette?
Igloo writes ..
The conventional ‘decomposition’ of vitamin C is in fact an oxidation. Details are given in another FAQ:
However, at temperatures higher than about 40 °C, the enzyme, ascorbic acid oxidase, will presumably be deactivated, and oxidation will only be able to take place without the help of this enzyme. It is quite possible that at higher temperatures the ascorbic acid molecule does break down (decompose) in some way, but I do not know how.
As to your second question, there is no reason why you shouldn’t have the orange juice exposed to air, certainly for relatively short periods of time. In any case oxygen can only make contact with the surface of the juice, unless it is being stirred vigorously.
The reason why we usually have the DCPIP solution in the burette is that the end point is established when the solution first turns pink/purple/blue. If the orange juice is in the burette, the DCPIP in the flask will of course be blue to start with, and the endpoint is reached when all traces of blue disappear. Since most of us find it easier to observe the first type of colour change, this is the way it is normally done. Also, it tends to be the case that we normally place the solution to be investigated in the flask, and have the reference solution in the burette.
In this case then you can carry out the titration either way.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
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updated: 17 December 2006
